Humidity resistant MoSx films prepared by pulsed magnetron sputtering

It is well known that MoS2 is a good lubricant, but the lubricity of MoSx thin films is greatly affected by the deposition parameters, especially when used under environmental conditions of high relative humidity. In this work, MoSx films were prepared by magnetron sputtering using a bipolar pulsed power supply. Several deposition parameters such as argon pressure, substrate temperature, substrate bias voltage, cathode power and deposition time were varied. Composition, morphology and structure were investigated by a number of techniques including energy dispersive spectroscopy (EDS), Rutherford back scattering (RBS), scanning electron microscopy (SEM) and X-ray diffraction (XRD). Tribological properties were measured with a ball-on-disk fretting tester. The results show that MoSx films prepared at low argon pressure (below 0.4 Pa) and low substrate temperature (room temperature) have a low friction coefficient and long wear life. These films have a remarkable low sulfur content (x≈1 and even smaller, in contrast to frequently reported values of x=1.2∼1.8), a featureless morphology and only a strong basal plane (002) diffraction peak. The relative humidity, up to values of 90%, has only a small effect on the friction coefficient and wear life. The structure of the films and the friction and wear mechanism are discussed in view of the low sulfur content.